The 5 human beta-type globin genes (e,Gg, Ag, d, and b) are regulated by the locus control region (LCR), which is a multimodular, multifunctional enhancer that acts from a considerable distance (>50-60kb 5') to enhance transcription. The sequential physical order of the beta-globin genes reflects their temporal expression during human development. The major controversy in the "hemoglobin switching" field concerns how each globin gene competes for the single LCR activity during development. Two models, the looping model and the tracking and linking model, have been proposed to explain hemoglobin switching. To elucidate the mechanisms of human beta globin switching, we propose to generate transgenic mice harboring a 200 kbp modified human beta globin YAC. Using site-specific Cre recombination in vivo and strategically placed loxP sites in the YAC, we will generate sibling transgenic mice lines harboring wild type human beta-globin locus or modified beta-gobin locus with the intervening region between LCR and adult beta globin genes deleted or replaced by neutral (lambda) DNA spacer at identical chromosomal integration site. Using these transgenic mice, we will ascertain the transcriptional consequences of placing the adult beta globin gene (1) in its wild type configuration, (2) juxtaposed to LCR, and (3) in its normal position but separated from the LCR by neutral DNA during primitive and definitive erythropoiesis. If the generally favored looping model is correct, then we anticipated that beta-globin gene transcripion will be equally or more robust in scenario (3) than (1). Further, the results of this experiment will likely lead to more research questions in beta globin gene regulation that I hope to address in future projects. This career development award will not only allow me to complete my proposed project but also build upon a young but promising career in the field of beta globin transcriptional regulation.